1. Field of the Invention
This invention relates to an improved guide device for cutting a groove or dado with a router, and more particularly to novel improvements in a guide by which a groove having a width that is larger than the diameter of a rotary bit to be used and the same as the thickness of a board can be easily formed by first spreading, with inserts of a predetermined thickness, a rectilinear guide member by a width that is the same as the diameter of the rotary bit. By then inserting a chip or scrap piece of the board to be fitted such as for shelving in the guide after making the first cut, the device allows a groove having the same width as the board to be fitted therein to be easily formed by a second cut.
2. Description of the Related Art
The work of forming grooves or dados in side boards of a bookshelf, for example, and fitting shelf board to the grooves has been performed in several ways, as shown, for example in FIG. 1. First, a groove 2 is formed in a side board 1 by a rotary bit (not shown) of a router. Then, a shelf board 3 is brought into contact with the groove 2 and a position P corresponding to the thickness of the shelf board 3 is marked by a pencil. After that, the groove 2 is cut again by the router so that the groove width increases to the position P. This conventional method of forming a dado described above had the following problems. To form a dado whose width is substantially the same as the thickness of a shelf board, the extra width not covered by the initial cut must be measured and marked in some fashion. Then the router must be run along the groove again to cut the dado to the desired width. Therefore, the work is complicated and it is difficult to cut a number of dados with an accurate width for the specific board to be used in each dado. To solve this problem, particularly when forming grooves that are larger than the rotary bit to be used, a variety of techniques using clamps and guide boards or fences have been proposed, but all suffer from being very complicated and inaccurate.
To overcome these problems, the present inventor developed, as disclosed in U.S. Pat. No. 5,738,470, a novel guide device for cutting a dado by which a groove having a width that is larger than the diameter of a rotary bit to be used and the same as the thickness of a board can be easily formed by defining, in a rectilinear guide member, an opening or a through hole having a width that is the same as the diameter of the rotary bit. By inserting a chip or scrap piece of the board to be fitted such as for shelving in the opening after making a first cut, the device allows a groove having the same width as the board to be fitted therein to be easily formed by a second cut. In this invention, a guide device for cutting a groove is provided which includes a rectilinear guide member for guiding a router provided with a rotary bit having a predetermined diameter, the rectilinear guide member comprising a pair of first and second guide members connected to be freely slidable in relation to each other and an opening formed between said guide members when they are in contact with each other having a width equal to a diameter of the rotary bit.
However, during development, the present inventor found a number of problems with this guide that made it very difficult to produce on a large scale at a cost acceptable to both professional and amateur woodworkers. One of the major problems involved the opening or through hole, which involves a variety of machining steps when the guide is made of a metal or a complicated mold when the guide is made of a plastic. It was also found that a through hole required that the guide be made of an initially thicker piece of material, so that after the through hole was machined, etc., the guide would still have enough strength to withstand use under relatively severe conditions while still possessing acceptable accuracy. This made the guide both bulky and relatively heavy. After numerous attempts to solve the above problem, he developed a number of designs that would allow more flexibility during use, even greater accuracy and increased strength, all of which required less complicated machining and a more efficient use of materials.
The present invention has been made with a view to solve the problems stated above and its object is to provide an improved guide device for cutting dados and more particularly to a novel improvement by which dados can be accurately and easily formed by using inserts of a predetermined thickness to replace the role of the opening or through hole in the previous guide.
According to this invention, a guide device for cutting a groove is provided which includes a rectilinear guide member for guiding a router provided with a rotary bit having a predetermined diameter, said guide member comprising a pair of first and second guide members which are connected to be freely slidable in relation to each other and which are urged into a normally closed state by at least one elastic band running through both the first and second guide members, and a set of inserts of at least one predetermined thickness, said inserts being inserted to spread the guide member by a width equal to a diameter of the rotary bit.